Much has happened to Marshall Mathers in the two-and-a-half years since his last solo album, The Eminem Show. He has been embroiled in a vicious war of words with another superstar rapper. He has been investigated by the CIA. He has been compared to Hitler, after a 16-year-old tape of him advising listeners to "never date a black chick" was discovered.

For any other artist, just one of these incidents would constitute a career-threatening disaster. By Eminem's standards, however, they amount to a stable and controversy-free few years. He has not been convicted of anything, nor denounced by the White House. The CIA concluded that he posed no threat to national security. The voice comparing him to Hitler belonged not to a human rights activist, but to Ray "Benzino" Scott, who was trying to jump-start his own ailing rap career and justify the racist coverage of Eminem in the Source, a magazine he co-owns. The war of words was with dunderheaded Ja Rule, which is a bit like pitting Alexander Pope against Pam Ayres. Even Mathers' reliably appalling mother finally seems to have shut up.

Perversely, his fourth album suggests the one thing that could constitute a career-threatening disaster for Eminem is relative peace and quiet. Ever since he escaped the trailer-park poverty featured on his 1999 debut The Slim Shady LP, Eminem has required regular supplies of fresh controversy and new targets. "He can't keep saying the same shit," remarked Proof, a rapper from Eminem's woeful posse D12, prior to the release of The Eminem Show. But he did, just more vociferously than before. The result was a commercially and artistically successful cul-de-sac. When you have recorded a track like Cleaning Out My Closet - which called his mother a "selfish fucking bitch" and hoped she would "burn in hell" - you have no further to go: there is no way to continue exploring hatred as a theme without becoming self-parodic.

To his credit, Eminem seems to realise that. Encore's opener Evil Deeds takes a half-hearted pop at his mother, but the subject is quickly forgotten. Puke attempts to make capital out of his ex-wife Kim's well-publicised drug addiction ("you're a fucking cokehead and I hope you fucking die" etc) but he sounds strained as he sings, as if he is trying to work up some bile even though his heart isn't in the task. In any case, sermonising about drugs is rather hard to take from a man with a Vicodin pill and a load of magic mushrooms tattooed on his arm.

To Encore's detriment, Eminem seems to have little idea as to what might replace the vitriol and existential angst. On My First Single and Just Lose It, his plan appears to involve belching and making fart noises, which, with the best will in the world, won't suffice. He makes jokes: some work (there's a good one about Christopher Reeve's able-bodied shadow waiting to beat Eminem up for making fun of his paralysis) but most are feeble, relying on the rapper's pyrotechnic delivery to make them work. There's no wit about telling his critics to ring "1-800-I'M-A-DICK-SUCKER", but the relish in his voice can make you laugh out loud. More often, he sounds bored, as if he's going through the motions. The flat and repetitive music follows suit - even Dr Dre's production lacks its usual inventive spark.

Three tracks demonstrate precisely what the rest of Encore is lacking, reminding you how remarkable Eminem can be when he has something to react against.

Yellow Brick Road addresses the "racist" mixtape, offering the unlikely sound of Eminem being reasonable. Forced to justify his place within the rap pantheon, he is at his most inventive, mixing autobiography with hip-hop history, finally apologising. Like Toy Soldiers, about Ja Rule and Benzino, is similarly brilliant. Set to the album's one genuinely fantastic backing track, involving a military drumbeat and a sample from Martika's forgotten 1980s hit Toy Soldiers, its lyrics switch from truce-calling to belligerent indignation and back again, often in the space of one line.

Finally, there is Mosh, the anti-war, anti-Bush track "leaked" just before the election. It offers both the best lyric Eminem has ever written and the one moment on the album where the repetitious production style works, providing a suitably relentless basis for his quickfire hectoring.

That Mosh seemingly did nothing to affect the election's outcome is something of a double-edged sword. Eminem has said all along that his lyrics are essentially harmless, but conversely, The Eminem Show's White America claimed his fans were like a vast army of young nihilistic discontents at his disposal. Perhaps they're too young to vote or too nihilistic even to do his bidding. In that case, they're unlikely to notice or care that much of Encore is lacklustre: another multi-platinum success seems a given.

But Eminem is nobody's fool: you suspect he knows perfectly well what the album's shortcomings are. His audible lack of interest during Encore's least-inspired moments raises intriguing questions about what happens next.